Adrenergic receptors play a major role in cardiovascular reactivity but have been largely neglected in reactivity studies. This study examines the association of beta- and alpha-adrenergic receptors, in combination with cardiovascular and neurohormonal measures, with cardiovascular reactivity. The study will follow two phases. Phase 1. Forty healthy male subjects will be exposed to a primarily beta-adrenergic stressor (public speaking) and a primarily alpha-adrenergic stressor (mirror star tracing). Heart rate, cardiac output, peripheral resistance, and blood pressure reactivity will be measured. We will then assess the association between these reactivity measures and resting measures of lymphocyte beta-adrenergic and platelet alpha-adrenergic receptor density and function, plasma norepinephrine and epinephrine, blood pressure, heart rate, cardiac output, and peripheral resistance. Subjects will be re-studied 6 weeks later in order to examine the test-retest stability of this relationship Phase 2. This phase of the study further examines the stability of the receptor-reactivity relationship. We will study subjects where adrenergic receptors and reactivity naturally undergo cyclic variation. Using the phase 1 protocol, forty normal cycling women will be tested twice: once during the follicular phase and once during the luteal phase of the menstrual cycle. This data will be compared to the data from phase 1 of the study. Keywords: adrenergic receptors, cardiovascular, behavioral medicine, stress, sympathetic nervous system, catecholamines, menstrual cycle, sex differences.